The following aspects of the growth and physiology of the higher plant Lemna have been clarified: (a) Three stages in the vegetative reproduction of Lemna were defined. Although all cultures contain colonies in these three stages of development, it was demonstrated that these cultures are essentially equivalent in terms of specific rates of net synthesis of methionine and other amino acids. (b) A growth medium for Lemna was developed which is as good (at high sulfate concentrations) or better (at low sulfate concentrations) than the most commonly used medium. (c) In the presence of the sulfate analogs molybdate or selenate, Lemna colonies have been maintained under conditions of sulfur limitation and with stable growth rates and protein contents for as long as 18 doublings. These analogs promise to be useful tools in our work. In Lemna we have previously demonstrated feedback regulation by methionine of de novo synthesis of its sulfur moiety, probably at the cystathionine synthesis step. Preliminary results suggest that methionine also feedback regulates de novo synthesis of its C4 moiety at the same locus. De novo synthesis of the methyl group tentatively appears to be regulated less than that of the C4 moiety of methionine.